Custom field not displaying using macro as default value

You probably need to revise your logic about alt field,i.e. when you form image tag for example in a transformation you can put there the default logic {%CurrentDocument.AltText ==""?CurrentDocument.Name +" picture":CurrentDocument.AltText|(identity)GlobalAdministrator%}. So essentially alt text will be empty except special cases.

I'd go with Peter's suggestion.

However, have you set the 'Has Depending Fields' on the FirstName field and the 'Depends on other fields' for the AltText? This would cause a postback when the firstname changes.

The proper syntax for the macro is

{%EditedObject.FirstName.Value|(identity)GlobalAdministrator%}

As somewhat explained in the previous answers, this won't always work, because on saving of the data, the macro may not know what the value is until later in the process. So best thing to do is as Suneel suggests, and set the Has depending fields on the FirstName field and Depends on other fields for the AltText. This will cause the postback and allow the AltText to populate with the FirstName value

Thank you for suggestions. Unfortunately it doesn't work. As I said in my initial post, I had has dependin fields on "FirstName" and depends on another field on AltText. Postback works but alt text is not updated. The macro displays only static part " picture". The macro provided by Brenden Kehren doesn't do the job either. (Value property is not detected by auto completion in macro editor)

{%EditedObject.FirstName.Value|(identity)GlobalAdministrator%}

I think It might not be possible and I really need to rethink my logic on this field as Peter suggested.

Just because the Value property is not auto detected doesn't mean it won't work. The proper way to access a field's value is to use FieldName.Value not FieldName. FieldName is the object.